Running Benchmark AHB2 in bridged mode and 4 Ohm Speaker

 I have AHB2, few month old.  In instruction manual rev G  p.22 it states:  

Output Current: 29 A peak into 1 Ohm, both channels driven 

The same is on their website (it used to be 18A).  Perhaps they improved power supply (higher limit).

Here you go: Page 26 "performance plots" fig. 1, 2, 3

https://cdn.shopify.com/s/files/1/0321/7609/files/AHB2_Manual_-_Rev_G.pdf?6581480924542235084

These plots show almost identical THD at 4 ohm, 8 ohm and bridged 8 ohm max power. As for damping factor - it does not make any difference, since it is already very high.

Quote from instruction manual:

The AHB2 power amplifier can be run in two modes: 

 STEREO mode - Two output channels 

 MONO mode - One output channel in bridged mono 

The bridged MONO mode will deliver almost 4 times as much power into an 8-Ohm speaker, and will do so without any increase in distortion. The MONO mode is an excellent choice for high-power applications. In MONO mode, two amplifiers will be required for stereo operation. 

40 amps of current is needed to produce 40 volts peak (100 watts RMS) across a 1 ohm dip in impedance. If this impedance is reactive the amplifier will be really unhappy.

I don't understand it.  40V across 1 ohm (or 40amps thru 1 ohm) would make 1600W. 

Note: Watts RMS means something different and has no relevance.  Power, obtained by multiplying  RMS current and RMS voltage is AVERAGE power (equal half of peak power for sinewave).  It represents DC power that would produce the same amount of heat.  RMS Power does not represent anything useful.

AHB2 power supply is highly regulated SMPS capable of producing large currents.  As for amp being less stable in bridged mode - AHB2 is designed to be stable since it does not have feedback in traditional sense.  Of course in order to reduce distortion that low it has to have negative feedback, but this feedback in not recursive, meaning it is not zapped back to the front of the amp.  Instead it drives another amp  (error amplifier), with another set of power transistors, that at the very end corrects output signal.

Also, low output impedance, doubled in bridged mode, might be important for driving complex loads, but it is already very low, about 0.03ohm @1kHz.  For the purpose of damping the speaker membrane it does not matter, since most of the speaker resistive impedance is in series anyway, reducing highest obtainable effective DF to 1.5

jon_5912, When you bridge 33.3V into 10ohm speaker you'll get 66.6V and 6.66A making 400W.  Bridging quadruples power (since it doubles the voltage).

It is also possible that 480W limitation comes from power dissipation of the output or power supply (29A max current seems adequate) and is not relevant, unless one listens to continuous sine waves.  Music has very low average power while peaks might still reach close to 800W.  

Maximum current won't double.  (It will be still 29A).    Maximum current is an absolute maximum that amplifier can deliver.  This specification shows amplifier's ability to drive difficult low impedance loads.  It says that when 4 ohm speaker's impedance drops to 1 ohm amp can deliver 29V peak.

Also, I'm not sure where specification 480W/4ohm came from.  My manual (version G), as well as their website, shows 480W/6ohm bridged and 130W/6ohm unbridged.  480W is not that far from theoretical 530W for ideal voltage source.

The maximum continuous current capability into 8, 4, whatever loads will increase when an amp is bridged.  

Agreed.  The only confusion is word "capability".  IMHO it should be:

The continuous current into 8, 4, whatever loads will increase when an amp is bridged.  

jon_5912, Yes, current will double when you double voltage, but you stated that current CAPABILITY (meaning max current) will be better. Current capability of AHB2 in bridged mode will be still 29A.

Assuming that you meant current (and not current capability) at given volume, it should state "at given volume knob position".  At given volume (meaning loudness) bridged and unbridged configuration currents will be the same.

At any given volume the bridged amp will have better current capability.

Current capability will be the same (29A for AHB2).  Bridging just doubles output voltage.

Power specified at 6ohm is 130W. Theoretical ideal power bridged at 6ohm should be 520W and is 480W (7.7% less). Also current was changed to 29A per channel. Perhaps 18A specification was for the early versions.

George, Where did you find 480W/4ohm? Their website states 480W/6ohm.

Ramtubes, AHB2 has load/line regulated SMPS, but as you mentioned, there are losses everywhere.  Exact doubling of power for half of the load means perfect regulation - an ideal voltage source, that is only possible with very deep negative feedback.  Such feedback might create instabilities or TIM distortions, but  feedback in AHB2 is quite different - it is non recursive, meaning that signal is not fed back to the input of the same amplifier but to another "error amplifier" that corrects output later.  Such arrangement eliminates instabilities (allowing for wide bandwidth of the amp) and TIM distortions.  I also noticed a lot of low bass added , in comparison to my previous class D amp (that had 5Hz low bandwidth limit).  My guess is that all DC servos have instability problems for very low frequencies, but AHB2, having no recursive feedback at all can go down to 0.4Hz.  Should 0.4Hz vs 5Hz difference be audible with 30Hz speakers? - I don't know, but it is in my setup (big surprise).  The same goes for the upper limit - more extension and airy treble.  Could this be phase delay at high frequencies with my class D amp?  Speaker itself has inductive character at higher frequencies accelerating phase.  In addition there are many amps, including one  of Lamm, that have lower (50kHz) bandwidth and are praised for wonderful airy highs.

Yes, drop across resistance will double, but reserves in power supply combined with the feedback will make up for that.

I dont see why it is a virtue especially if it is just a spec and not what is really going on. Has Stereophile tested any of these amps. Please provide a link.

I never said it is a virtue - just the opposite.

Once an amp clips feedback cannot make up for these losses.

I agree, but amp that doubles power exactly has to have adequate power supply AND deep negative feedback.  Exact doubling implies perfect regulation (ideal voltage source), that is not possible without deep feedback.

Ramtubes, I explained exactly what I agreed with by quoting your statement - read again.

Why are you bringing emitter resistors again, yes we know that they exist. 

I'm not sure why you're asking me what Benchmark voltage regulation is.   Perhaps you should direct this question to John Siau.

What does the deep feedback have to do with it?

Feedback lowers output impedance.  You cannot make exact doubling of the power if DF=10.

If you can site an amplifier that exactly doubles bring it on. Show me.

There are many of them (I'm surprised you don't know that).  For instance Rowland model 535 is specified for 250W@8ohm and 500W@4ohm.  Krell Evolution 302e specifies 300W@8ohm 600W@4ohm and 1200W@2ohm (both channels driven).  Pass Labs XS150 states 150W@8ohm 300W@4ohm  etc.

You keep asking "what feedback has to do with it" while I'm trying to show you, that exact doubling of power cannot be achieved without deep feedback.  Exact doubling of the power is treated on this forum as a virtue, while I don't think it is necessary a good thing and might be even harmful when deep feedback is used to spike-up amplifier's  specifications while creating TIM distortions at the same time.

I think, we are talking about two different things.  I have never mentioned actual amplifier's power at clipping but rather manufacturer's specifications (that are quite different), that calls for power doubling (or close to it). 

I have also noticed personal comments from you, placing my math ability in question and calling me to learn basic electronics.  I believe that I have some comprehension of basic electronics designing it for 40 years and therefore I view your comments as harmful, therefore refusing to comment on anything you have to say from this point on.  Perhaps one day, when you change your attitude we can start talking again.

Ramtubes, PEACE!  I'm sorry for my comments.  It was uncalled for.  I have short fuse (electrical term :) ) recently, since I completely lost hearing in the right ear few months ago (replaced by loud white noise - so much for Benchmark's fantastic S/N ratio).  I will probably stay for a while, learning and possibly helping, but I slowly loose interest in this hobby.  Am I the perfect candidate for bridging AHB2 (only one channel needed)?  Again peace, man.

Thank you Al, but I lost all hope - it is almost 6 months now and no improvement.  All audio tests show -95dB.  My doctor says it was most likely a virus from the shingles variety.  After 15 sessions in hiperbaric chamber and two direct injections of steroids thru the eardrum my eyes suddenly improved and  I'm typing this without glasses (+2 before).  Doctors have no clue why.

Thank you hifidream, mapman